Electroluminescent storage device



March 14, 1961 E. E, LOEBNER ETAL 2,975,291

ELECTROLUMINESCENT STORAGE DEVICE Filed OCl'f. 17, 1957 e ze sra/afanarran/HUMA@ WMS EBUN E. LDEBNER,

HARVEY GII-Imm 5 DDNALD CDARLINE MM gw ELECTROLUMINESCENT STGRAGE DEVICE'Egon E. Loebner and Harvey 0. Hook, Princeton, NJ., and Donald E.Darling, Levittown, Pa., assignors to Radio Corporation of America, acorporation of Dela- Ware Filed Oct. 17, 1957, Ser. No. 690,814 v 9Claims. (Cl. Z50-213) This invention relates to electroluniinescentdevices and particularly to solid state devices designed to amplify andstore radiation images, such as light images.

Solid state light amplifying and storage devices are known whichcomprise layers of photoconductive and ielectroluminescent phosphormaterial sandwiched between two transparent sheet electrodes. When analternating voltage is applied to the electrodes, the two layersfunction as a voltage divider. Without illumination or irradiation ofthe photoconductive layer, the division of voltage is such that by farthe greater portion appears across the photoconductive layer while theelectroluminescent `layer receives only a small voltage which isinsuicient to cause appreciable light emission therefrom. In thepresence of an incident image, such as light or X-rays, for instance,the impedance of the photoconductive layer lowers, area by area, inaccordance with the radiation intensity Variations in the image so thatcorrespondingly greater voltage appears, area by area, across theelectroluminescent layer. These Voltage variations result in variationsin light emission from the electroluminescent layer which, for arelatively wide range of incident radiation intensities, correspond tobut are greater than the variations in the incoming image. The image isthus intensified.

Such a device can store an image indefinitely if a sufficient amount ofthe electroluminescent light is made to feed back to the photoconductivelayer to keep it excited. Modified structures have also been designedspecilically for storage purposes. However, rnost of the prior artstructures do not have high storage efiiciency, or lack ne definition,or are difficult to fabricate.

It is therefore a primary object of this invention to provide animproved structure, for use in a storage light amplifier, which iseasily fabricated.

Another object is to provide a storage light amplifier of very linedefinition and high feedback efficiency.

'Ihe above and other objects are achieved, in accordance with thisinvention, by providing a plate of insulating material formed withlaterally spaced transparent portions integrally surrounded by opaqueportions. The transparent portions are provided with apertures extendingthrough the plate and containing bodies of photoconductive material. Thebodies are respectively connected electrically to electroluminescentphosphor capacitor areas disposed adjacent to one side of the plate.Each transp-arent portion of the plate serves as a light duct for theillumination of the photoconductive body therein both by incident lightand by light fed back from the electroluminescent area registeredtherewith. The opaque portions serve to absorb or reliect suicientfeedback 4light to prevent the undesirable triggering of neighboringimage elements into storage and thus cause a smearing and spreading ofthe image. A mosaic of mutually isolated transparent conductive areas isarranged as interlayer electrodes between the transparent portions andthe phosphor areas in such fashion as t-o permit the n 2,975,291?atented Mar. 14, 1961 electroluminescent light emission to be directedmost eiciently through the transparent light ducts and onto the surfacesof the photoconductive bodies.

In the drawings:

Fig. 1 is a fragmentary sectional view of a storage light amplifieraccording to the invention; and

Fig. 2 is a fragmentary perspective view showing a portion of the deviceof Fig. 1 enlarged and in greater detail.

Referring to Fig. l, the storage light amplifier panel comprises anum-ber of layers which include a first conductive coating 10, anapertured insulating plate 12, a mosaic of conductive patches 26, anelectroliuninescent Vphosphor layer 14, and a second conductive coating16,

in that order. Bodies 18 of pho-toconductive material are contained inthe apertures of openings Zit of the plate 12. The first conductivecoating l@ is transparent to the input radiations, and the secondconductive coating 16 is transparent to the output radiations.

As shown in greater detail in Fig. 2, the -apertured plate 12,preferably of glass, is made up of a great many transparent volumes orlight ducts 22 optically separated from one another by a network ofrelatively narrow opaque portions of light baflies 24. The plate 12 -isa unitary structure, with -the opaque portions 24 prei erably beingformed by darkening the original glass of which the plate 12 iscom-posed. This process is described in greater detail later. Theapertures 2@ extend through the light ducts 22 from side to side of theplate 12. One side of the plate 12 is covered with the continuoustransp-arent conductive coating 10, such as a thin film of tin oxide,gold, or aluminum. The coating 10 extends, a slight distance intotheapertures 2% to form a narrow conductive band on the inner wall ofeach aperture. Alternatively the photoconductor body may extend a slightdistance outward and beyond the apertures to providea sufficiently largecontact area between the photoconductor and the coating 1i). The otherside of the plate 12 is coated with the mosaic of mutually insulatedconductive patches 26, which are transparent to the feedback radiation.The patches 26 coat the top surfaces of the light ducts 22 and, like thecoating 10, extend into the apertures 2t) to form narrow conductivebands or they overlap somewhat the photoconductive body filling theapertures. In one particular method of laying down this mosaic, anetwork of grooves 28 is first etched into the glass and the grooves arefilled with a substance which serves as a mask. The transparentconductive material is then produced on the masked plate, by spraying orotherwise, after which the masking substance is removed from the plate,leaving only the patches 26 coated on the surface of the plate 12.

The apertures 20 are filled with the bodies or plugs 18 ofphotoconductive material, such as cadmium sulfide o1' cadmium selenidecrystalline powder. The powder may be applied dry or it may be mixedwith a suitable dielectric binder such as ethyl cellulose or an epoxyresin. rI'he ends of the plugs 18 make good conductive contact with theconductive coatings 1@ and patches 26.

The layer 14 of electroluminescent phosphor is applied over the patches26, and the phosphor layer 14 in turn is coated with transp-arentconductive material, suc-h as a thin film of gold, forming the secondconductive coating 16. The electroluminescent phosphor may lill in thegrooves 2S. Alternatively, the grooves may first be filled in with inertmaterial, such as a resin, which is insulating and preferably opaque,and the electroluminescent phosphor may be subsequently applied.Electroluminescent phosphor materials are well known, and in general amaterial is chosen which closely matches the spectral response of thephotoconductive material of the plugs 13. Photoconductive cadmiumsulfide, copper and chlorine activated, has a maximum green to redresponse, so that a green emitting phosphor, such as copper and aluminumactivated zinc sulfide can be used with that material. Theelectroluminescent material is preferably mixed with a transparentdielectric binder material such as et-hyl cellulose or an epoxy resin.`

ln operation, a source 30 of alternating voltage, such as, for example,800 volts at 200 cycles per second frequency, is connected to theconductive coatings 1t) and 16 which serve as external electrodes. Inthe absence of an input image on the light amplifier, most of theelectric field is developed across the photoconductive plugs 1S and onlya small threshold iield appears across the phosphor layer 14, as is wellknown. When a light or radiation image is projected on the panel fromeither side thereof, the incoming light rays irradiate the sides of Ithephotoconductive plugs 18, rendering the surfaces more conductive so thata greater field is built up across the adjacent portion of phosphorlayer 14, whereupon the latter electrolurninesces. In the areas of thepanel where the incoming light is relatively intense, the plugs 18become more conducting and there is a greater eld built up across thephosphor areas adjacent thereto, so that the light emission from thesephosphor areas is relatively high. In the darker areas, the plugs 18 areless conductive, the electric field across the phosphor areas is less,and the light emission is less. In this way, a mosaic of variableintensity areas of a light image is produced and amplilied element byelement.

ri`he transparent conductive patches 26 of the mosaic serve to spreadthe currents uniformly over the adjacent areas of the phosphor layer 14covered thereby. Furthermore, because of their location on the lightducts 2?., they tend to direct back into the light ducts some of theelectrolluminescent light so as to establish feedback and light storage.

It has been shown that it is possible to adjust operating conditions,i.e., voltage and frequency of the power source, of a single opticalpositive feedback light intensilier cell so that it may have two stableoperating states, one which is light emitting and the other which isessentially non-light-emitting. The ability to store indefinitely eitherof the two states gives rise to the term storage light amplifier. In animage intensifying panel some light unavoidably seeps through, orcross-feeds, to olf cells. Indefinite image storage is achieved when theintensity of this cross-feed light is kept below a certain triggerthreshold of the exposed oil cells. Above this threshold, image storagetime is finite, and the speed of spreading is related to the amount oflight exposure of the oil-cell photoconductor.

The width of the light ducts 22 is made rather large compared to thewidths both of the photoconductive plugs 1S and the light battles 24, sothat a substantial amount of light, either input or feedback, or both,can illuminate the photoconductive plugs 18. The light batilles 24 formcompartments surrounding each light duct 22 and plug 1S so as to confinethe light within each compartment. in this way, cross-lumination betweenadjacent elements and spreading of the stored light pattern isprevented.

More than one photoconductive plug may be disposed Within the confinesof each light baille. For instance, it may be desired to locate a trioof such plugs, each responsive to dillerent color light, eg. red, greenand blue, with a single bathe, with each plug being responsive tofeedback light from a corresponding one of three different coloremitting electroluminescent phosphor elements.

It is pointed out that the terms transparent and opaque as applied tothe light ducts 2,2 and baliies 24 are used in a relative sense withreference to the feedback light and photoconductor sensitivity. Since,it is only the feedback light which is involved in storage operation,and the feedback light may be a different color than the input light,the light bailes need only be suticiently opaque to the feedback lightand then only to that feedback light to which the photoconductor isresistive to prevent triggering of normally dark photoconductive plugs.

To fabricate the plate 12 several known techniques can be used. Thus, itis well known that certain materialsV like alkali halides aretransparent to visible radiation when -in a virgin state, but discolorand become non-transmitting in a number of wavelength ranges whenexposed to X-ray or similar radiation. An opaque network is obtained ina plate of alkali halide material, such as potassium chloride orpotassium fluoride, by bringing it in contact with a mask consisting ofa network of X-ray transparent, i.e. low atomic number, material such aslithium, boron or graphite, filled with X- ray opaque, i.e. high atomicnumber, material such as lead. The alkali halide plate is then exposedto preferably collimated X-radiation through the X-ray mask. Suchcollimated radiation can be simulated by scanning the plate with afinite X-ray source, however, keeping satisfactory separation betweensource and mask and providing a suitable thickness of the mask. Insteadof a conventional X-ray source, which has to be scanned, a gamma-rayemitting sheet of radio-active material can be used to expose thepattern through the above described mask.

The apertures can be obtained either by ultra-sonic machining, otherphysical machining, or by physicochemical means described below. Thus,radiation from a heat lamp is allowed to fall through a focusedprojection system onto the glass plate, thereby producing a heat imageof the desired pattern on the plate. A suitable solvent is applied tothe plate, the temperature of the solvent being kept well below that atwhich rapid dissolving action takes place. However, at the focused heatimage areas the temperature rises well above the dissolving threshold.Thus, the differential dissolving rate between the heated and cooledparts of the solvent and the plate will result in the production of thedesired apertures. It should be pointed out that the formation of theapertures be preferably done before the formation of the opaque networkto avoid bleaching of the opaque network by the application of heat.

In another process which utilizes photographically sensitive glass knownas Fotoform glass, the Fotoform glass is exposed to ultra-violet lightlirst through a photographic master, with clear areas in the mastercorresponding to the desired positions of the apertures. These areas arethen developed by heating the structure to a suitable temperature, about450 C. until the aperture areas become opalescent. One side of the glassis then ground and polished to provide a suitable surface for a secondexposure, this time to the light baille pattern. To this end, a secondphotographic master, with clear areas corresponding to the desiredopaque areas in the glass, is registered with the previously developedaperture pattern on the glass, and a suitable exposure to ultravioletlight ismade. The aperture pattern is then etched through withhydrofluoric acid, the exposed areas being more soluble in the acid thanthe clear areas. Next, the bailile pattern is developed by heating theplate to about 550 C. although higher temperatures up to 650 C. may beused. The structure is then ground to size and polished on both sides.The batlles are then etched on one side 0f the plate with hydrofluoricacid to obtain grooves. The grooves and apertures are then lled with asuitable masking material, such as Alundum, and both sides of thestructure coated with a transparent conductor. The masking material isthen removed, leaving the mosaic of conductive patches.

It is thus apparent by means of the invention, a storage light amplifierof simple construction and having fine definition and'high feedbacketlciency is provided.

What is claimed is:

1. A light-responsive structure for use in a storage light ampliercomprising a plate of glass of substantial thickness which hasintegrally formed therein a network of opaque portions dividing saidplate into a multiplicity of optically separated transparent portions,each of said transparent portions having an opening therethrough, and atleast one solid photoconductive body in each of said openings.

2. A structure comprising a plate of.solid insulating material formedwith an array of laterally spaced transparent portions each integrallysurrounded by an opaque portion, each of said transparent portionshaving an opening extending therethrough and a solid photoconductivebody in each of said openings.

3. A structure comprising a plate of solid insulating material formedwith an array of laterally spaced transparent portions each integrallysurrounded by an opaque portion, each of said transparent portionshaving an opening extending therethrough and a solid photoconductivebody in each of said openings, and a layer of conducting material on oneside of said plate registered with each of said transparent portions andconnected to each of said photoconductive bodies.

4. An electroluminescent device comprising a plate of solid insulatingmaterial formed with an array of laterally spaced transparent portionseach integrally surrounded by an opaque portion, a layer ofelectroluminescent phosphor adjacent to one side of said plate andcomprising elemental areas -exposed to the other side of said platethrough said transparent portions, and solid photoconductive areassupported by said plate so that each photoconductive area is exposed toa corresponding one of said phoshor areas through one of saidtransparent portions but is optically shielded from all other phosphorareas o by said opaque portions.

5. A storage light amplifier comprising an integral support plate formedof solid insulating material which includes an array of spacedtransparent portions surrounded by opaque portions, electroluminescentphosphor areas adjacent to one side of said plate, and solidphotoconductive plugs within said transparent portions supported by saidplate so that each photoconductive plug is optically coupled to acorresponding one of said phosphor areas through said transparentportions but is optically shielded from all other phosphor areas by saidopaque portions, said optically coupled photoconductive plugs andphosphor areas being electrically connected together.

6. A storage light amplifier comprising a plate of solid insulatingmaterial which includes an array of apertured transparent portions eachintegrally surrounded by an opaque portion, electroluminescent phosphorareas adjacent to one side of said plate and exposed to the other sidethrough said transparent portions, and solid photoconductive bodiessupported in the apertures of said plate and each having surfaceportions which are exposed to a corresponding one of said phosphor areasbut optically shielded from all other phosphor areas.

7. A storage light amplier comprising a glass plate integrally formedwith a network of opaque portions of the glass forming compartments withtransparent portions of the glass therein, a solid photoconductive plugwithin each compartment extending from side to side of said plate andsurrounded by one of said transparent portions, a mosaic of mutuallyinsulated transparent conductive patches on one surface of said plateand registered with said compartments and connected respectively to saidplugs, a layer of electroluminescent phosphor on said mosaic, conductivemeans on said phosphor layer, and conductive means on the other side ofsaid glass plate, at least one of said last two conductive means beingtransparent to light.

8. A storage light amplier as in claim 7, wherein the volume oftransparent glass is substantially greater than the volume of opaqueglass.

9. A storage light amplifier as in claim 7, wherein the volume oftransparent glass is substantially greater than the volume of saidphotoconductive plugs.

References Cited in the le of this patent UNITED STATES PATENTS SheldonAug. 13, 1957 UNITED STATES PATENT OEEICE CERTIFICATE OF CORRECTIONPatent No. 2,975,291- March I4, 1961 Egon E. Loeloner et al.

vIt is hereby certified that error appears in the above numbered patentrequiring correction and that the said Letters Patent should read ascorrected below.

In the grant, line 2, and in the heading to the printed specification,line 4, name 01' the third inventor, for Donald E. Darling", eachoccurrence, read Donald C.

Signed and sealed this 17th day of October 1961.,

(SEAL) Attest:

ERNEST W. SWTDEE DAVID L. LADD Commissioner of Patents USCOMM-DCAttesting Officer

